Mapping Mountains – Hill Reclassifications – 30-99m Twmpau

Ynys Fach (SM 707 237) – Double Sub-Twmpau addition

There has been an addition to the list of 30-99m Twmpau, with the summit height and its location, the drop and status of the hill confirmed by LIDAR analysis conducted by Aled Williams.

Ynys Fach (SM 707 237)

The criteria for the list that this addition applies to are:

30-99m Twmpau - Welsh hills at or above 30m and below 100m in height with 30m minimum drop, this height band of hills has two accompanying sub lists, the first of which is entitled the 30-99m Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 30m and below 100m in height with 20m or more and below 30m of drop, and the second sub category is entitled the Double Sub-Twmpau with the criteria for this sub category being all Welsh hills at or above 20m and below 30m in height with 20m or more and below 30m of drop, with the word Twmpau being an acronym standing for thirty welsh metre prominences and upward.

The name of the hill is Ynys Fach, and as its name implies it is an island, and it is adjoined to the Carn Llidi group of hills which are situated in the south-western part of Mid and West Wales (Region B, Sub-Region B4), and is positioned adjacent to the central eastern coastline of Ynys Dewi.

This hill did not appear in the accompanying Hills to be surveyed sub list when the original Welsh 30-99m P30 hills were published on Geoff Crowder’s v-g.me website, as it did not meet the criteria then used for this sub category, however this sub list has now been standardised, with interpolated drop values and interpolated summit heights also included.

It was not until LIDAR became available that the details for this hill could be accurately assessed.  The LIDAR (Light Detection & Ranging) technique produced highly accurate height data that is now freely available for much of England and Wales.

LIDAR image of Ynys Fach (SM 707 237); 1m contour (black), 10m contour (red) and sea level (yellow)

The addition of this hill to Double Sub-Twmpau status is due to LIDAR analysis conducted by Aled Williams, resulting in a 25.7m summit height and with its bwlch at sea level this is also the drop value for the hill, which is sufficient for it to be classified as a Double Sub-Twmpau.

The full details for the hill are:

Group:  Carn Llidi

Name:  Ynys Fach

OS 1:50,000 map:  157

Summit Height:  25.7m (LIDAR)

Summit Grid Reference:  SM 70712 23702 (LIDAR)

Bwlch Height:  N/A, sea level

Bwlch Grid Reference:  N/A, sea level

 

Drop:  25.7m (LIDAR)

My thanks to Aled Williams for forwarding his LIDAR analysis of this hill

Myrddyn Phillips (July 2019)

© Crown: CHERISH PROJECT 2019. Produced with EU funds through the Ireland Wales Co-operation Programme 2014-2020. All material made freely available through the Open Government Licence.